Rapid detection of isthmus block and rhythm change using local electrogram changes during complex atrial flutter ablation

Abstract Aims Multiple re-entry circuits may operate simultaneously in the atria in the form of dual loop re-entry using a common isthmus, or multiple re-entrant loops without a common isthmus. When two or more re-entrant circuits coexist, ablation of an individual isthmus may lead to a seamless transition (without significant changes in surface electrocardiogram, coronary sinus activation or tachycardia cycle length) to a second rhythm, and the isthmus block can go unnoticed. Methods and results We hypothesize and subsequently illustrate in three patient cases, methods to rapidly identify a transition in the rhythm and isthmus block using local electrogram changes at the ablation site. Conclusion Local activation sequence changes, electrogram timing, and the behaviour of pre-existing double potentials can reveal isthmus block promptly when rhythm transitions occur during ablation of multiloop re-entry tachycardias.

What's new? (1) Multiloop atrial tachycardias may exist in the form of figure of eight re-entry with a common isthmus or individual simultaneous re-entries without a common isthmus. (2) During multiloop atrial re-entry tachycardias, ablation of a noncommon isthmus (NCI) can lead to unrecognized isthmus block and rhythm change.
(3) Local activation sequence changes, electrogram timing, and the behaviour of pre-existing double potentials can instantaneously identify block at an NCI during complex atrial flutter ablation. (4) Prompt recognition of the local electrogram changes indicating block can help avoid unnecessary continued ablations at an already blocked isthmus and potentially add to procedural safety.
Multiple re-entry circuits may operate simultaneously in the atria in the form of dual loop re-entry using a common isthmus, or multiple reentrant loops without a common isthmus. When two or more reentrant circuits coexist, ablation of an individual isthmus may lead to a seamless transition [without significant changes in surface electrocardiogram (ECG), coronary sinus activation, or tachycardia cycle length (TCL)] to a second rhythm, and the isthmus block can go unnoticed. It is important to recognize an isthmus block promptly to avoid prolonged unnecessary ablations at an already blocked site. Electroanatomic remapping of the entire circuit to identify rhythm changes is time consuming and difficult to do frequently. Repeat entrainment mapping could be challenging due to difficulty with capture thresholds and decremental conduction in areas of scar. We describe methods to rapidly identify a transition in the rhythm using local electrogram changes at the ablation site.

Hypothesis
In a re-entrant circuit, the local electrograms at an isthmus may be considered in terms of the depolarization wavefront approaching the site (approaching wavefront, Awf) and the wavefront receding from the site (receding wavefront, Rwf) ( Figure 1). When two simultaneous circuits are operative, the rhythm can be a figure of eight (dual loop) re-entry 1 involving a common isthmus (CI) (Figure 1) or two independent re-entrant circuits with separate isthmuses ( Figure 2). Even in a dual loop re-entry, there may be additional independent isthmuses that are unique only to one of the re-entrant loops [non-common isthmus (NCI)]. Ablation of the common isthmus will result in termination of a dual loop re-entry, whereas ablation of the NCI would result in local block and transition of the rhythm to the second re-entry. 2 We hypothesized that when conduction block occurs at a NCI, the re-entry circuit would change with alteration of local activation at the site.
The following would indicate rhythm change and isthmus block: (1) Activation sequence: Reversal of activation sequence at the Rwf side, with or without reversal of activation at the Awf side of ablation line. Any of these changes would indicate that the direction of activation at the isthmus being ablated has changed. Change in the direction of activation implies a change in the rhythm, most likely a result of isthmus block at the site of ablation. In the case of a dual loop re-entry, block at the NCI will lead to reversal of activation only at the Rwf ( Figure 1). In the case of two rhythms without a common isthmus, ablation of an individual isthmus could lead to change of activation on the Rwf and Awf ( Figure 2). The exact changes would depend on the location and direction of activation from the second re-entry, and the nature and location of the isthmus. Finally, re-entry around a defined scar typically features wide double potentials (DPs) due to temporal separation of activation wavefronts on either side of the line of block. Merging of these DPs during ablation denotes change of activation at the site and isthmus block. Three representative cases presented below demonstrate these principles. Supplemental files are provided including video propagation maps and complete lesion sets of the three cases, and procedural details of Case 1.

Case 1: Dual loop re-entry (mitral + roof flutter)
A 72-year-old man presented with recurrent atypical flutter after previous pulmonary vein isolation and typical cavo-tricuspid isthmus (CTI)-dependent flutter ablations. Electroanatomic mapping of the presenting rhythm showed dual loop re-entry with counterclockwise mitral re-entry and roof-dependent re-entry around the left pulmonary veins ( Figure 3A). Ablation lesions between left and right superior pulmonary   Figure 1. In Panel B, note that the activation changes at the Rwf and Awf areas with block in the NCI of one of the circuits. A narrow and confined isthmus is depicted, and the wavefront must enter it at the edges leading to a reversal of Rwf area activation. If the isthmus is broad, it could be activated without reversal at the Rwf or Awf, but relative timing would still change.
veins posteriorly (roof line) led to a 10 ms slowing of the TCL (230-240 ms). Surface ECG and coronary sinus activations remained unchanged ( Figure 4). It was unclear if the roof line blocked or whether the small increase in TCL was due to conduction delay. The activation sequence at the Rwf was continuously being evaluated during ablation with a multipolar catheter (HD grid™, Abbott Laboratories), and a reversal of the activation wavefront was noted coinciding with the TCL change ( Figure 3C). This reflected a change in rhythm and block across the roof line. The mitral activation was delayed by 10 ms because the roof re-entry was the faster loop in this case. However, the reversal of activation at the Rwf would have been indicative of rhythm change and isthmus block even without TCL change. Further electroanatomic mapping confirmed a counterclockwise mitral flutter with no further roof reentry ( Figure 3B). A mitral isthmus ablation between the left inferior pulmonary vein and mitral valve terminated the tachycardia and differential pacing at the roof and mitral lines indicated block at both sites.

Case 2: Two re-entrant circuits without a common isthmus
A 52-year-old man with previous history of atrial septal defect (ASD) repair presented with ECG morphology suggestive of typical CTI flutter. Electroanatomic mapping revealed re-entry around an atriotomy scar anterior to the SVC ( Figure 5A). The TCL was 285 ms, and wide DPs were apparent with blue isochrones adjacent to white isochrones at the mid part of the scar. No simultaneous re-entry was noted around the tricuspid valve (TV), but a potential lower loop clockwise re-entry around the inferior vena cava was noted. An ablation line was performed from TV to the atriotomy scar. No tachycardia termination, change in TCL, change in coronary sinus (CS) activation, or change in surface ECG occurred despite anatomic completion of the ablation line. However, the wide DPs seen previously at the atriotomy site were no longer seen and replaced by closely spaced electrograms  ( Figure 5C). Conversion of DPs to closely spaced electrogram was interpreted as indicative of a rhythm change and block of the isthmus being ablated. Additionally, the Rwf electrogram advanced (blue to red isochrone) and Awf electrogram advanced (blue to orange isochrone), both indicating a rhythm change. Remapping of the rhythm was undertaken revealing no further scar re-entry but continued lower loop re-entry around the inferior vena cava ( Figure 5B). A CTI ablation terminated the lower loop re-entry.

Case 3
A 74-year-old man presented for ablation of typical CTI flutter and 3D electroanatomic map demonstrated dual loop re-entry with counterclockwise typical flutter and intra-isthmus re-entry around a scar midway in the CTI. The common isthmus of the dual loop was the part of the CTI posterior to the scar and the NCI was the part of the CTI anterior to the scar. Ablation was started at the TV end of the CTI, and wide DP formation was noted during ablation of the CTI anterior to the scar (Figure 6) without change in TCL, ECG morphology, or CS activation. The timing of the first component of the DP was the same as previous electrograms at the site while the second component was delayed either from block or conduction delay at the site. Further electroanatomic mapping was not undertaken and completion of the CTI ablation posterior to the scar (common isthmus) terminated the tachycardia. Bidirectional CTI block was confirmed.

Discussion
In this report, we characterize methods for quick identification of rhythm change and isthmus block during ablation of complex atrial flutters. We propose that conduction block at an NCI during multiloop re-entry would result in identifiable activation changes at the site. Figure 1 shows the expected changes when a NCI of a dual loop reentry is ablated. These changes are demonstrated in patient Case 1 where mitral and roof flutters form a dual loop re-entry. With ablation of the roof (NCI), the Rwf activation is reversed from the initial pattern, but the rest of the left atrial activation remained unchanged from the mitral re-entry. A reversal of activation can be identified with either a multipolar mapping catheter instantaneously (as we did in Case 1) or using the roving ablation tip to map a limited area at the Rwf. When a stable multipolar mapping catheter position at the Rwf area is difficult, the catheter may be moved to the area as needed to observe electrogram sequence and timing. The ablation tip electrograms provide a continuous guide for electrogram timing changes at Rwf or Awf areas. If high density mapping of the initial rhythm has been undertaken, one can predict the area where Rwf activation reversal would occur from the line of collision of activation wavefronts during the original dual loop re-entry. 2 In Case 1, one could predict that the Rwf electrogram timing will change to immediately past the reference (light blue on isochronal map) from before the reference (red on isochronal map) when block occurs. Here, one can watch for the Rwf electrograms delaying to the blue isochrone during ablation as likely indicative of block. The timing of the Awf remains unchanged in Case 1 despite the increase in TCL because the CS (reference electrode) activation delays equally and the relative timing of electrograms are unaffected.
In the case of two re-entrant circuits without a common isthmus, block in one isthmus transitions the rhythm to the second re-entry ( Figure 2). The exact changes vary depending on the relative locations of the circuits and isthmuses. Reversal of activation of the Rwf is expected if the isthmus is anatomically confined because this area will now be activated from a different wavefront in the opposite direction. If the area is not confined, reversal of activation may not occur, but the timing may advance or delay ( Figure 2B). In addition to the Rwf activation changes, the Awf activation could also change here. These changes are demonstrated in patient Case 2. Any change (advancement or delay) of the Awf electrogram would indicate rhythm change because ablation of the isthmus should not affect the activation of areas temporally preceding it unless the area is activated differently. It is not necessary to demonstrate activation reversal when changes occur in the Awf-one can use a single electrogram, and advancement or delay would be sufficient to show rhythm change. Similarly, any advancement of Rwf electrograms will also indicate a rhythm change because ablation induced slowing of conduction can only delay the Rwf electrograms. The scar re-entry in Case 2 also illustrates the usefulness of observing DP timing. Merging of the DP electrogram along the scar indicates termination of the re-entry around the scar and a rhythm change. A comparison with Case 3 illustrates the difference between DP merging vs. DP formation during ablation. The former, when confirmed along the scar, indicates rhythm change, whereas the latter can be rhythm change or conduction slowing. It should be noted that rhythm change during ablation of an isthmus could also occur with revelation of a re-entrant circuit or a focal tachycardia that was not apparent before. The activation changes we describe here are applicable to occurrence of such unexpected new rhythms.
While the local electrogram changes we describe here should prompt the electrophysiologist to re-evaluate the need for continued ablation at the site, it should be used as a guide rather than definitive evidence of isthmus block. Achievement of block does not mean that the ablation at the site should be immediately discontinued. There may be value for further ablation and completion of the anatomic line depending on the stage of ablation. While the changes at the Rwf perhaps provides the most information during an ablation, reversal of Rwf activation may not always indicate block at an isthmus. Significant delay with Rwf reversal can occur due to focal breakthrough conduction on the ablation line or epicardial conduction with breakthrough distant from the line of ablation. Ultimately, Rwf reversal should be taken as likely to indicate block and the state of isthmus conduction further confirmed after rhythm termination.

Conclusion
Local activation sequence changes, electrogram timing and the behaviour of pre-existing DPs can detect isthmus block when rhythm transitions occur during ablation of multiloop re-entry tachycardias. Expeditious identification of isthmus block, that otherwise may have gone unnoticed, could improve patient safety and outcomes.